1. Field of the Invention
The present invention relates to an ink composition favorably used for inkjet recording, an inkjet recording method, and a printed material produced therewith.
2. Description of the Related Art
There are many image-recording methods of forming an image on a recording medium such as paper based on image data signals, including those in an electrophotographic process, a sublimation or fusion heat-transfer process, and an inkjet process. Among them, the inkjet process is advantageous in that it allows printing in an inexpensive device, and because it forms an image directly on a recording medium by ejecting ink only in a required image region, ink can be used more efficiently, and running costs are low. In addition, the inkjet process is also less noisy and thus advantageous as an image-recording method. The inkjet process allows printing not only on plain paper but also on non-water absorbing recording media such as a plastic sheet or metal plate, but it is imperatively required to increase the printing speed and improve the image quality. The time required for drying and curing the ink droplets after printing significantly influences the efficiency in producing printed materials and sharpness of the printed image.
Among the inkjet recoding processes, there is a recording method by using an inkjet recording ink that can be cured by irradiation with a radiation ray. In the method, it is possible to improve the printing efficiency and form a sharp image, by curing the ink droplet by irradiating it with a radiation ray immediately or after a particular period from ink ejection. By increasing the sensitivity of the inkjet recording ink which is curable by irradiation with a radiation ray such as an ultraviolet ray, the ink has higher curability with respect to radiation rays, which brings about many benefits such as improvement in inkjet recording efficiency, reduction in power consumption, prolongation of the lifetime of a radiation ray generator owing to the decrease in the load, and prevention of volatilization of low-molecular substances caused by insufficient curing. Further, the increase in the sensitivity particularly increases the strength of the image formed using the inkjet recording ink. In particular, a planographic printing plate produced using the ink has increased strength in the image portion thereof, and thus provides higher printing durability.
In recent years, such an inkjet recording process using an inkjet recording ink that cures by irradiation with a radiation ray, such as an ultraviolet ray, has been increasingly drawing attention due to relatively low odor, quick-drying properties, and the ability to record on a target recording medium having no ink absorption properties, and a cationic polymerization ink composition having excellent adhesiveness to a target recording medium and low shrinkage at the time of ultraviolet curing has been proposed (e.g., Japanese Patent Application Laid-Open (JP-A) No. 9-183928). However, the cationic polymerization inks have insufficient stability during storage due to a reaction caused by acid generated with time, which has become a serious hindrance in practical utilization thereof. Therefore, a technique of adding a basic compound or a thermal base generator has been proposed as an attempt to improve storage stability (e.g., Japanese Patent Application Laid-Open (JP-A) Nos. 2003-312121, 2003-341217, and 2004-91558). However, it has been revealed that there arises another problem in that a basic compound impairs functions of acid generated by light exposure, which decreases the curing sensitivity of the ink.